1. Field of the Invention
The present invention relates to a method for controlling an oxygen concentration sensor, and more particularly to a method for controlling a sensor for sensing an oxygen concentration in an exhaust gas of an internal combustion engine.
2. Description of Background Information
In order to accelerate the purification of the exhaust gas and to improve the fuel economy of an internal combustion engine, a feedback type air/fuel ratio control system is generally used, in which oxygen concentration in the exhaust gas is detected and air/fuel ratio of the mixture supplied to the engine is controlled to a target air/fuel ratio by a feedback control operation in accordance with a result of the detection of the oxygen concentration.
As an oxygen concentration sensor for use in such an air/fuel ratio control system, there is a type which is capable of producing an output signal whose level is proportional to the oxygen concentration in the exhaust gas of the engine in a region in which the air/fuel ratio of the mixture is larger than a stoichiometric air/fuel ratio, and the detail of which is disclosed in Japanese patent application laid open No. 58-153155. This oxygen concentration sensor includes an oxygen concentration sensing unit having a general construction including a pair of flat solid electrolyte members having oxygen ion permeability. These oxygen ion conductive solid electrolyte members are placed in the exhaust gas of the engine, and electrodes are respectively provided on the front and back surfaces of both of the solid elctrolyte members. In other words, each pair of electrodes sandwich each solid electrolyte member. These two solid electrolyte members each haing a pair of electrodes are arranged in parallel so as to face each other and forming a gap portion, or in other words, a restricted region between them.
With this arrangement, one of the solid electrolyte members serves as an oxygen pump element and the other one of the solid electrolyte members serves as a sensor cell element for sensing an oxygen concentration ratio. In an ambient atmosphere of the exhaust gas, a drive current is supplied across the electrodes of the oxygen pump element in such a manner that the electrodes facing the gap portion operates as a negative electrode. By the supply of this current, i.e. a pump current, the oxygen component of the gas in the gap portion is ionized on the surface of the negative electrode of the oxygen pump element. The oxygen ions migrate through the inside of the oxygen pump element to the positive electrode, where the oxygen ions are released from the surface thereof in the form of the oxygen gas.
While this movement of the oxygen ions is taking place, the oxygen concentration becomes different for the gas in the gap portion and the gas outside the sensor cell element because of a decrease of the oxygen gas component in the gap portion. Therefore, a voltage develops across the electrodes of the sensor cell element. If the magnitude of the pump current supplied to the oxygen pump element is controlled so that the voltage generated across the sensor cell element is maintained constant, the magnitude of the pump current varies substantially in proportion to the oxygen concentration in the exhaust gas under a condition of a constant temperature. The pump current is then used as an output signal indicative of the oxygen concentration detection value.
By means of the magnitude of the pump current supplied to the oxygen pump element, a detection as to whether the air/fuel ratio of the mixture supplied to the engine is rich or lean is performed. In the case of the air/fuel ratio control system in which the air/fuel ratio is controlled by the supply of the air intake side secondary air, the secondary air is supplied when the air/fuel ratio is detected to be rich. On the other hand, the supply of the secondary air is stopped when the air/fuel ratio is detected to be lean. Thus the air/fuel ratio is controlled toward a target air/fuel ratio by the supply and stop of the air intake side secondary air.
In this type of oxygen concentration sensor, if an excessive current is supplied to the oxygen pump element, it causes the so called blackening phenomenon by which the oxygen ions are removed from the solid electrolyte members. For instance, when zirconium dioxide (ZrO.sub.2) is used as the solid electrolyte, the oxygen ions O.sub.2 are taken from the zirconium dioxide (ZrO.sub.2) so that zirconium (Zr) is separated out. As a result of this blackening phenomenon, a deterioration of the oxygen pump element takes place rapidly, to cause a debasement of the operation of the oxygen concentration sensor as a whole.
FIG. 1 shows curves indicating a current I.sub.P to the oxygen pump element versus oxygen concentration relation and a boundary line of the occurence of the blackening phenomenon. As illustrated, magnitude of the current I.sub.P varies in proportion to the oxygen concentration, and the rate of variation is different for several different values of the voltage V.sub.s developing across the electrodes of the sensor cell element. In other words, the voltage Vs is a parameter which determines the relation between the magnitude of the current I.sub.P and the oxygen concentration. As illustrated in this figure, the boundary line of the occurence of the blackening phenomenon is shown, as in the case of the magnitude of the current I.sub.P, as a first-degree function of the oxygen concentration value. Therefore, for preventing the blackening phenomenon, it is necessary that the magnitude of the supply current to the oxygen pump element is limited to be smaller than values in the region of the blackening phenomenon.
Further, in this type of the oxygen concentration sensor, it is necessary that the operating temperature of the sensing unit is sufficiently higher (for example, higher than 650.degree. C.) than an exhaust gas temperature under a steady state operation, in order to obtain a proportional output signal characteristic in which the sensor output signal varies substantially in proportion to the oxygen concentration. To meet this requirement, a heater element which is made up of a heater wire, for example, is incorporated in the oxygen concentration sensing unit and a drive current is supplied to the heater element at the time of measurement so that heat is generated at the heater element.
When the supply of the heater current is started upon a cold start of the engine, the oxygen concentration sensing unit remains inactive because the temperature of the oxygen concentration sensing unit does not rise immediately to a level at which the desired proportional output signal characteristic is obtaned. On the other hand, after a hot start of the engine, the temperature of the oxygen concentration sensing unit reaches to the level at which the sensing unit is activated, within a relatively short time. Therefore, the detection of the activation of the oxygen concentration sensing unit can not be easily performed simply by using a time period elapsed after the start of the engine. Further, since the feedback control of the air/fuel ratio in response to the outout signal of the oxygen concentration sensor is enabled only after the completion of the activation of the oxygen concentration sensing unit, it is important to detect the activation of the oxygen concentration sensing unit accurately for the air/fuel ratio control operation. If the completion of activation of the oxygen concentration sensing unit is not detected accurately, the feedback control of the air/fuel ratio is not started at a proper time after the start of the engine, and an open loop control of the air/fuel ratio is performed before the start of the feedback control. Therefore, it has not been always possible to attain an optimum efficiency of the purification of the exhaust gas by means of conventional apparatuses.